internal/task, runtime: add subsections_via_symbols to assembly files on darwin

This allows the assembly routines in these files to be stripped as dead
code if they're not referenced.  This solves the link issues on MacOS
when the `leaking` garbage collector or the `coroutines` scheduler
are selected.

Fixes #2081

src/internal/task/task_stack_amd64.S

@@ -72,3 +72,8 @@ tinygo_swapTask:
 
     // Return into the new task, as if tinygo_swapTask was a regular call.
     ret
+
+#ifdef __MACH__ // Darwin
+// allow these symbols to stripped as dead code
+.subsections_via_symbols
+#endif

src/runtime/gc_amd64.S

@@ -27,3 +27,8 @@ _tinygo_scanCurrentStack:
     // were only pushed to be discoverable by the GC.
     addq $56, %rsp
     retq
+
+#ifdef __MACH__ // Darwin
+// allow these symbols to stripped as dead code
+.subsections_via_symbols
+#endif